The ability to communicate with each other is a fundamental part of our lives and work environment. Since the advent of wireless communication devices we have grown accustomed to being able to communicate whenever and wherever we are. This of course has huge benefits but as the number of people communicating in this manner grows quickly this sets a tough demand on the communication infrastructure to follow suite. People using these wireless communication possibilities require a certain degree of quality and safety with their communication sessions.
Public safety groups, such as police, rescue personnel, and similar task forces also have a desire to communicate with each other with good quality and preferably on secure links on wireless communication channels, for instance at a location where a situation is under way where the task forces are operating.
So called Push-to-talk services over cellular access is seen as a solution to Public Safety group communication. There are a number of issues to overcome, which of one is the very high requirements on capacity, in particular the requirement on a very large number of users in one single cell.
A solution may be to transmit the voice information via radio multicast, similar to existing solutions for legacy Public Safety systems. Multimedia Broadcast Multicast Service (MBMS) is a solution, standardized in 3GPP for cellular systems.
This is a short summary of basic MBMS GERAN (GSM EDGE Radio Access Network) functionality.                The downlink common data stream of an MBMS session originates from a server called BM-SC (Broadcast Multicast Service Center).        One data stream per involved GGSN, SGSN and BSC (Base Station Controller).        An MBMS service context is set up per BSC initiated by the message MBMS SESSION START REQUEST received from SGSN. The BSC shall extract the following information contained in the message: Temporary Mobile Group Identity (TMGI), MBMS Session Identity (optional), MBMS service Area Identity List, Estimated Session Duration and Guarantied Bit Rate.        MBMS sessions are notified to capable mobiles by a special PAGING REQUEST message containing among others TMGI and MBMS Session ID (Identity).        MBMS bearers (Packet Data Channels) are uniquely established per session and cell by a BSS MBMS assignment procedure invoked by MBMS service requests from mobiles belonging to the right TMGI.        
There are some problems with the current standardized solution of MBMS.                Uplink Packed Data Channels reserved for an MBMS session can only be used for the signaling needed to maintain the downlink data stream, e.g. ACK/NACK messages etc.        A mobile may not be in packet transfer mode when receiving an MBMS session according to the 3GPP standard of today.        The new standard for PS (packet Switched) handover (3GPP Rel 7) is not planned to be used for MBMS.        